1. Field of the Invention
This invention relates to fiber-reinforced ceramics suitable for structural parts, etc.
2. Description of the Prior Art
Attempts to apply ceramics to structural parts, mechanical parts, etc. as substitutes for metals have been actively made by virtue of characteristics of ceramics, such as a high heat resistance, a high oxidation resistance, a high strength, a high attrition resistance, etc. Among the ceramics, particularly silicon-based ceramics such as Si.sub.3 N.sub.4, Si.sub.6-z Al.sub.3 O.sub.z N.sub.8-z (0&gt;z.ltoreq.4), SiC, etc. have been regarded as important owing to a high strength at an elevated temperature and a distinguished oxidation resistance, but have such a disadvantage as a high brittleness.
To overcome the brittleness of ceramics and improve the fracture toughness, methods for dispersing fibers of high tenacity into the ceramics to give the ceramics fiber reinforcement have been proposed. The methods are particularly effective for glass matrix, as disclosed in J. J. Brennan; K. M. Prewo, J. Mater. Sci. 17 2371-2383 (1982), and materials whose fracture toughness exceeds 15 MN/m.sup.3/2 in terms of K.sub.lc have been known. However, the materials based on the glass matrix generally have a lower heat resistance than the ceramics, and have such a disadvantage that they are deformed under a load at an elevated temperature for a prolonged time.
On the other hand, fiber-reinforced ceramics have not had a sufficiently high fracture toughness, as disclosed, for example, in Hibashi: Sunshine Journal Vol. 3, No. 4 (1982), pages 20-28, and have had a problem in the mechanical reliability.
SiC fiber-reinforced ZrO.sub.2, mullite, and cordierite are reported in B. A. Bender Proc. of 8th Annual Conference on Composites and Advanced Ceramic Materials, pages 513-529, January 15 to 18 (1984). Some of these ceramics have a relatively high fracture toughness, but their strength is about 80 to about 180 MPa, which is rather small for the ceramics. Furthermore, it seems that since ceramics are oxides, their strength will be lowered by softening at an elevated temperature. It is mentioned in the said report that there is a reaction phase in a film state composed of carbon at the boundaries between the 30 wt. % BN-containing mullite and the SiC fibers, where the reaction phase does not contribute to an improvement of toughness and thus the toughness of the ceramics is low.
Furthermore, it is mentioned in the same report that the SiC fiber-reinforced glass has a low strength, and even if it has a high strength, the strength will be lowered by softening at an elevated temperature. That is, the desired ceramics having a high strength at an elevated temperature and a high toughness for structural materials have not been available.
Examples of utilizing whiskers are disclosed in Japanese Patent Application Kokai (Laid-open) No. 59-54680, where ceramics less susceptible to strength deterioration up to a high temperature can be obtained by integrating SiC whiskers throughout Si.sub.3 N.sub.4. However, the toughness of ceramics cannot be considerably improved only by the whiskers, mainly because distribution of whiskers becomes uneven if 25% by volume or more of whiskers are contained within the ceramic matrix, and consequently the density is considerably lowered. That is, normal sintered products cannot be obtained. This is also because the diameters of commercially available SiC whiskers obtained according to a gas phase process are as small as 0.5-1.0 .mu.m and are not so different from the particle size of the ceramic matrix, and consequently the ability of the whiskers to absorb the cracking is low.
It is also reported in M. W. Lindly; D. J. Godfrey: Nature 229, 192-193 (1971) that Si.sub.3 N.sub.4 ceramics reinforced by silicon carbide fibers having diameters of 80 to 100 .mu.m have a relatively large fracture energy, but have a strength of 127 MPa (about 13 kg/mm.sup.2), which is low for the ceramics, because Si.sub.3 N.sub.4 is sintered according to a reaction sintering process and pores remain pores in the Si.sub.3 N.sub.4. In this case, W wires chemically vapor-deposited (CVD) with SiC are used as fibers, and it seems that W reacts with SiC when the ceramics are used at an elevated temperature for a long time, deteriorating the fiber characteristics.